/* let moduleA = { // 使用 1 2 3
  providers: [1, 2, 3]，
  exports: [1, 2]
}
let moduleB = { // 可以使用 1 2 4 5
  imports: [moduleA],
  providers: [4, 5],
  exports: [moduleA]
}

let moduleC = { // 1 2
  imports: [moduleB]
} */

let innerModule = {
  providers: [InnerService, AddService],
  exports: [InnerService]
}
let commonModule = {
  imports: [innerModule],
  providers: [CommonService, AddService],
  exports: [CommonService, innerModule]
}
let coreModule = {
  imports: [CommonModule],
  providers: [],
  exports: [CommonModule] // 此处导出的可以是提供者，也可以是模块
}
let AppModule = { // CommonService InnerService
  imports: [CoreModule] // 此处导入的只能是模块
}